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Linux/net/unix/garbage.c

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  1 /*
  2  * NET3:        Garbage Collector For AF_UNIX sockets
  3  *
  4  * Garbage Collector:
  5  *      Copyright (C) Barak A. Pearlmutter.
  6  *      Released under the GPL version 2 or later.
  7  *
  8  * Chopped about by Alan Cox 22/3/96 to make it fit the AF_UNIX socket problem.
  9  * If it doesn't work blame me, it worked when Barak sent it.
 10  *
 11  * Assumptions:
 12  *
 13  *  - object w/ a bit
 14  *  - free list
 15  *
 16  * Current optimizations:
 17  *
 18  *  - explicit stack instead of recursion
 19  *  - tail recurse on first born instead of immediate push/pop
 20  *  - we gather the stuff that should not be killed into tree
 21  *    and stack is just a path from root to the current pointer.
 22  *
 23  *  Future optimizations:
 24  *
 25  *  - don't just push entire root set; process in place
 26  *
 27  *      This program is free software; you can redistribute it and/or
 28  *      modify it under the terms of the GNU General Public License
 29  *      as published by the Free Software Foundation; either version
 30  *      2 of the License, or (at your option) any later version.
 31  *
 32  *  Fixes:
 33  *      Alan Cox        07 Sept 1997    Vmalloc internal stack as needed.
 34  *                                      Cope with changing max_files.
 35  *      Al Viro         11 Oct 1998
 36  *              Graph may have cycles. That is, we can send the descriptor
 37  *              of foo to bar and vice versa. Current code chokes on that.
 38  *              Fix: move SCM_RIGHTS ones into the separate list and then
 39  *              skb_free() them all instead of doing explicit fput's.
 40  *              Another problem: since fput() may block somebody may
 41  *              create a new unix_socket when we are in the middle of sweep
 42  *              phase. Fix: revert the logic wrt MARKED. Mark everything
 43  *              upon the beginning and unmark non-junk ones.
 44  *
 45  *              [12 Oct 1998] AAARGH! New code purges all SCM_RIGHTS
 46  *              sent to connect()'ed but still not accept()'ed sockets.
 47  *              Fixed. Old code had slightly different problem here:
 48  *              extra fput() in situation when we passed the descriptor via
 49  *              such socket and closed it (descriptor). That would happen on
 50  *              each unix_gc() until the accept(). Since the struct file in
 51  *              question would go to the free list and might be reused...
 52  *              That might be the reason of random oopses on filp_close()
 53  *              in unrelated processes.
 54  *
 55  *      AV              28 Feb 1999
 56  *              Kill the explicit allocation of stack. Now we keep the tree
 57  *              with root in dummy + pointer (gc_current) to one of the nodes.
 58  *              Stack is represented as path from gc_current to dummy. Unmark
 59  *              now means "add to tree". Push == "make it a son of gc_current".
 60  *              Pop == "move gc_current to parent". We keep only pointers to
 61  *              parents (->gc_tree).
 62  *      AV              1 Mar 1999
 63  *              Damn. Added missing check for ->dead in listen queues scanning.
 64  *
 65  *      Miklos Szeredi 25 Jun 2007
 66  *              Reimplement with a cycle collecting algorithm. This should
 67  *              solve several problems with the previous code, like being racy
 68  *              wrt receive and holding up unrelated socket operations.
 69  */
 70 
 71 #include <linux/kernel.h>
 72 #include <linux/string.h>
 73 #include <linux/socket.h>
 74 #include <linux/un.h>
 75 #include <linux/net.h>
 76 #include <linux/fs.h>
 77 #include <linux/skbuff.h>
 78 #include <linux/netdevice.h>
 79 #include <linux/file.h>
 80 #include <linux/proc_fs.h>
 81 #include <linux/mutex.h>
 82 #include <linux/wait.h>
 83 
 84 #include <net/sock.h>
 85 #include <net/af_unix.h>
 86 #include <net/scm.h>
 87 #include <net/tcp_states.h>
 88 
 89 /* Internal data structures and random procedures: */
 90 
 91 static LIST_HEAD(gc_inflight_list);
 92 static LIST_HEAD(gc_candidates);
 93 static DEFINE_SPINLOCK(unix_gc_lock);
 94 static DECLARE_WAIT_QUEUE_HEAD(unix_gc_wait);
 95 
 96 unsigned int unix_tot_inflight;
 97 
 98 struct sock *unix_get_socket(struct file *filp)
 99 {
100         struct sock *u_sock = NULL;
101         struct inode *inode = file_inode(filp);
102 
103         /* Socket ? */
104         if (S_ISSOCK(inode->i_mode) && !(filp->f_mode & FMODE_PATH)) {
105                 struct socket *sock = SOCKET_I(inode);
106                 struct sock *s = sock->sk;
107 
108                 /* PF_UNIX ? */
109                 if (s && sock->ops && sock->ops->family == PF_UNIX)
110                         u_sock = s;
111         }
112         return u_sock;
113 }
114 
115 /* Keep the number of times in flight count for the file
116  * descriptor if it is for an AF_UNIX socket.
117  */
118 
119 void unix_inflight(struct user_struct *user, struct file *fp)
120 {
121         struct sock *s = unix_get_socket(fp);
122 
123         spin_lock(&unix_gc_lock);
124 
125         if (s) {
126                 struct unix_sock *u = unix_sk(s);
127 
128                 if (atomic_long_inc_return(&u->inflight) == 1) {
129                         BUG_ON(!list_empty(&u->link));
130                         list_add_tail(&u->link, &gc_inflight_list);
131                 } else {
132                         BUG_ON(list_empty(&u->link));
133                 }
134                 unix_tot_inflight++;
135         }
136         user->unix_inflight++;
137         spin_unlock(&unix_gc_lock);
138 }
139 
140 void unix_notinflight(struct user_struct *user, struct file *fp)
141 {
142         struct sock *s = unix_get_socket(fp);
143 
144         spin_lock(&unix_gc_lock);
145 
146         if (s) {
147                 struct unix_sock *u = unix_sk(s);
148 
149                 BUG_ON(!atomic_long_read(&u->inflight));
150                 BUG_ON(list_empty(&u->link));
151 
152                 if (atomic_long_dec_and_test(&u->inflight))
153                         list_del_init(&u->link);
154                 unix_tot_inflight--;
155         }
156         user->unix_inflight--;
157         spin_unlock(&unix_gc_lock);
158 }
159 
160 static void scan_inflight(struct sock *x, void (*func)(struct unix_sock *),
161                           struct sk_buff_head *hitlist)
162 {
163         struct sk_buff *skb;
164         struct sk_buff *next;
165 
166         spin_lock(&x->sk_receive_queue.lock);
167         skb_queue_walk_safe(&x->sk_receive_queue, skb, next) {
168                 /* Do we have file descriptors ? */
169                 if (UNIXCB(skb).fp) {
170                         bool hit = false;
171                         /* Process the descriptors of this socket */
172                         int nfd = UNIXCB(skb).fp->count;
173                         struct file **fp = UNIXCB(skb).fp->fp;
174 
175                         while (nfd--) {
176                                 /* Get the socket the fd matches if it indeed does so */
177                                 struct sock *sk = unix_get_socket(*fp++);
178 
179                                 if (sk) {
180                                         struct unix_sock *u = unix_sk(sk);
181 
182                                         /* Ignore non-candidates, they could
183                                          * have been added to the queues after
184                                          * starting the garbage collection
185                                          */
186                                         if (test_bit(UNIX_GC_CANDIDATE, &u->gc_flags)) {
187                                                 hit = true;
188 
189                                                 func(u);
190                                         }
191                                 }
192                         }
193                         if (hit && hitlist != NULL) {
194                                 __skb_unlink(skb, &x->sk_receive_queue);
195                                 __skb_queue_tail(hitlist, skb);
196                         }
197                 }
198         }
199         spin_unlock(&x->sk_receive_queue.lock);
200 }
201 
202 static void scan_children(struct sock *x, void (*func)(struct unix_sock *),
203                           struct sk_buff_head *hitlist)
204 {
205         if (x->sk_state != TCP_LISTEN) {
206                 scan_inflight(x, func, hitlist);
207         } else {
208                 struct sk_buff *skb;
209                 struct sk_buff *next;
210                 struct unix_sock *u;
211                 LIST_HEAD(embryos);
212 
213                 /* For a listening socket collect the queued embryos
214                  * and perform a scan on them as well.
215                  */
216                 spin_lock(&x->sk_receive_queue.lock);
217                 skb_queue_walk_safe(&x->sk_receive_queue, skb, next) {
218                         u = unix_sk(skb->sk);
219 
220                         /* An embryo cannot be in-flight, so it's safe
221                          * to use the list link.
222                          */
223                         BUG_ON(!list_empty(&u->link));
224                         list_add_tail(&u->link, &embryos);
225                 }
226                 spin_unlock(&x->sk_receive_queue.lock);
227 
228                 while (!list_empty(&embryos)) {
229                         u = list_entry(embryos.next, struct unix_sock, link);
230                         scan_inflight(&u->sk, func, hitlist);
231                         list_del_init(&u->link);
232                 }
233         }
234 }
235 
236 static void dec_inflight(struct unix_sock *usk)
237 {
238         atomic_long_dec(&usk->inflight);
239 }
240 
241 static void inc_inflight(struct unix_sock *usk)
242 {
243         atomic_long_inc(&usk->inflight);
244 }
245 
246 static void inc_inflight_move_tail(struct unix_sock *u)
247 {
248         atomic_long_inc(&u->inflight);
249         /* If this still might be part of a cycle, move it to the end
250          * of the list, so that it's checked even if it was already
251          * passed over
252          */
253         if (test_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags))
254                 list_move_tail(&u->link, &gc_candidates);
255 }
256 
257 static bool gc_in_progress;
258 #define UNIX_INFLIGHT_TRIGGER_GC 16000
259 
260 void wait_for_unix_gc(void)
261 {
262         /* If number of inflight sockets is insane,
263          * force a garbage collect right now.
264          */
265         if (unix_tot_inflight > UNIX_INFLIGHT_TRIGGER_GC && !gc_in_progress)
266                 unix_gc();
267         wait_event(unix_gc_wait, gc_in_progress == false);
268 }
269 
270 /* The external entry point: unix_gc() */
271 void unix_gc(void)
272 {
273         struct unix_sock *u;
274         struct unix_sock *next;
275         struct sk_buff_head hitlist;
276         struct list_head cursor;
277         LIST_HEAD(not_cycle_list);
278 
279         spin_lock(&unix_gc_lock);
280 
281         /* Avoid a recursive GC. */
282         if (gc_in_progress)
283                 goto out;
284 
285         gc_in_progress = true;
286         /* First, select candidates for garbage collection.  Only
287          * in-flight sockets are considered, and from those only ones
288          * which don't have any external reference.
289          *
290          * Holding unix_gc_lock will protect these candidates from
291          * being detached, and hence from gaining an external
292          * reference.  Since there are no possible receivers, all
293          * buffers currently on the candidates' queues stay there
294          * during the garbage collection.
295          *
296          * We also know that no new candidate can be added onto the
297          * receive queues.  Other, non candidate sockets _can_ be
298          * added to queue, so we must make sure only to touch
299          * candidates.
300          */
301         list_for_each_entry_safe(u, next, &gc_inflight_list, link) {
302                 long total_refs;
303                 long inflight_refs;
304 
305                 total_refs = file_count(u->sk.sk_socket->file);
306                 inflight_refs = atomic_long_read(&u->inflight);
307 
308                 BUG_ON(inflight_refs < 1);
309                 BUG_ON(total_refs < inflight_refs);
310                 if (total_refs == inflight_refs) {
311                         list_move_tail(&u->link, &gc_candidates);
312                         __set_bit(UNIX_GC_CANDIDATE, &u->gc_flags);
313                         __set_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags);
314                 }
315         }
316 
317         /* Now remove all internal in-flight reference to children of
318          * the candidates.
319          */
320         list_for_each_entry(u, &gc_candidates, link)
321                 scan_children(&u->sk, dec_inflight, NULL);
322 
323         /* Restore the references for children of all candidates,
324          * which have remaining references.  Do this recursively, so
325          * only those remain, which form cyclic references.
326          *
327          * Use a "cursor" link, to make the list traversal safe, even
328          * though elements might be moved about.
329          */
330         list_add(&cursor, &gc_candidates);
331         while (cursor.next != &gc_candidates) {
332                 u = list_entry(cursor.next, struct unix_sock, link);
333 
334                 /* Move cursor to after the current position. */
335                 list_move(&cursor, &u->link);
336 
337                 if (atomic_long_read(&u->inflight) > 0) {
338                         list_move_tail(&u->link, &not_cycle_list);
339                         __clear_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags);
340                         scan_children(&u->sk, inc_inflight_move_tail, NULL);
341                 }
342         }
343         list_del(&cursor);
344 
345         /* Now gc_candidates contains only garbage.  Restore original
346          * inflight counters for these as well, and remove the skbuffs
347          * which are creating the cycle(s).
348          */
349         skb_queue_head_init(&hitlist);
350         list_for_each_entry(u, &gc_candidates, link)
351                 scan_children(&u->sk, inc_inflight, &hitlist);
352 
353         /* not_cycle_list contains those sockets which do not make up a
354          * cycle.  Restore these to the inflight list.
355          */
356         while (!list_empty(&not_cycle_list)) {
357                 u = list_entry(not_cycle_list.next, struct unix_sock, link);
358                 __clear_bit(UNIX_GC_CANDIDATE, &u->gc_flags);
359                 list_move_tail(&u->link, &gc_inflight_list);
360         }
361 
362         spin_unlock(&unix_gc_lock);
363 
364         /* Here we are. Hitlist is filled. Die. */
365         __skb_queue_purge(&hitlist);
366 
367         spin_lock(&unix_gc_lock);
368 
369         /* All candidates should have been detached by now. */
370         BUG_ON(!list_empty(&gc_candidates));
371         gc_in_progress = false;
372         wake_up(&unix_gc_wait);
373 
374  out:
375         spin_unlock(&unix_gc_lock);
376 }
377 

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